Normal EEG findings

The posterior dominant rhythm (PDR) is a well-formed, quite symmetric, sinusoidal/rhythmic waveform best observed in the posterior head regions, namely in the occipital channels. The PDR is best seen during wakefulness when eyes are closed/upon eye closure during relaxation or relative mental inactivity, and is attenuated by eye opening.

Please note that the PDR frequency normally progresses as patients age, as shown in the table below. In adults, the normal frequency is in the alpha range.

Transitions in state of consciousness result in changes in frequency, distribution, and amplitude of the dominant background activity. The onset of drowsiness is marked by a decrease in alpha activity and an increase in centrally predominant polymorphic theta activity (5-8 Hz), which should not exceed 30% of the recording in the adult awake state.

Horizontal slow roving eye movements may be present during drowsiness. They are seen as out-of-phase waveforms in the anterior leads (Fp1-F7; F7-T3 on the left and Fp2-F8; F8-T4 on the right). The slow roving eye movements can also be seen as undulating waveforms in the LOC-ROC channel where electrodes are placed on the left outer canthus (LOC) and right outer canthus (ROC).

Vertex sharp waves are surface-negative, spiky/sharp waveforms with maximal negativity at the vertex (Cz). They are normal EEG graphoelements, typically seen during light sleep. They are typically isolated, but trains of repetitive vertex waveforms may occur.

Positive occipital sharp transients of sleep (POSTS) have a biphasic morphology (reverse check mark), are surface-positive and occipital, and often appear in brief, semi-rhythmic trains up to 4-5 Hz. They are usually synchronous but asymmetric. POSTS appear during light sleep (stages N1 and N2) and may persist into slow-wave sleep but are virtually never seen during REM sleep.

Sleep spindles appear as bursts of 11 to 16 Hz sinusoidal activity predominantly in the midline and parasagittal regions during light sleep. They first appear around the age of 2 months. These are typically asynchronous until 18-24 months of age after which they become largely synchronous. Sleep spindles are the EEG hallmark of stage N2 sleep although they can also occur during stage N3 sleep. They are hypothesized to be produced by synchronous cortical and thalamic activity during non-REM sleep.

K-complexes are high-amplitude, negative deflections that are frontally predominant and followed by a smaller positive deflection as well as admixed sleep spindles. They are typically seen during stage N2 sleep.

During stage N3 sleep, the majority of each EEG epoch is comprised of continuous, generalized, polymorphic/irregular, high-amplitude (>75 uV), low-frequency (predominantly 1-2Hz delta) waveforms. These are intermixed with spindle activity, and muscle artifact and eye blinks are absent.

Rapid eye movement (REM) sleep is characterized by a desynchronized EEG pattern with mixed frequencies in the theta to beta range and reduced muscle activity. A characteristic feature of REM sleep is rapid eye movements which are seen in the frontal leads and can be accompanied by a low-voltage spiky waveform called the lateral rectus spike.

Sawtooth waves, appearing as 2-6 Hz asymmetric, monomorphic waveforms with a serrated appearance, may be present during REM sleep and should not be mistaken for epileptiform activity.